Many commercial and industrial processes require precise knowledge of the physical properties of substances being used in those processes in order to operate efficiently. For example, it is important to accurately determine the rheological properties, such as viscosity, of a liquid in food, cosmetics, paint, ink and lubricant industries. For example, controlling the viscosity of inks is very important for the proper performance of high speed printers. Also, the performance of paint sprayers and similar high speed paint application equipment requires that the paint viscosity be accurately controlled. In general, as industrial processes become more accurate and made to operate at higher speed, an accurate assessment of the viscosity of liquids used in the process is more and more important.
The viscosity of a fluid is a measure of the fluid's resistance to flow. Several commercially available conventional devices (viscometers) measure the viscosity of a fluid sample by rotating the sample within a container. A disk is suspended within the fluid sample and connected to a measurement apparatus. The viscosity of the fluid causes the suspended disk to rotate. The device calculates the viscosity of the fluid by measuring the amount that the suspended disk rotates, since the rotational force imposed on the disk by the rotating fluid is proportional to the fluid's viscosity.
In order to measure the rotation of the suspended disk, conventional viscometers incorporate mechanical components, such as a coiled spring, to determine the angular displacement of the disk. Measurements made with these types of systems, however, are subject to errors. For example, errors can result from the nonlinear characteristics of the spring caused by manufacturing imperfections, and from variations in spring tension due to temperature changes. It is common for the accuracy of these mechanical viscometers to be about one part in 100 or about 1%.
Additionally, the limited dynamic range of the spring requires that the viscometer be calibrated using a calibration standard that has a viscosity closely approximately the viscosity of the sample being measured. As is readily apparent, since the sample's viscosity is not known, the selection of the calibration standard is subject to error.
Another problem with mechanical spring-type designs is that the springs are typically designed to measure small forces. As such, these devices can be easily damaged if an excessive force is applied.
Other types of viscometers currently available operate by rotating a disk within a fluid sample in a stationary container. The drag imposed on the disk by the fluid sample is proportional to the viscosity of the fluid sample. These types of viscometers suffer similar deficiencies as with the viscometers which rotate the fluid sample.
A need, therefore, exists for an improved rotational viscometer which provides accurate viscosity measurements for a fluid sample and is easy to calibrate.